Hyperbranched polyethylenes are amorphous (extensive short- and long-chain branching prevent polyethylene segment crystallization) and exhibit many useful properties. Hyperbranched polyethylenes can be used as lubricant base stocks or as viscosity index improvers in lubricants or as viscosity modifiers in hydrocarbon fluids. Hyperbranched polyethylenes exhibit favorable anticipated temperature invariant coil dimension, shear and oxidative stability, and incompressibility at high pressure. The hyperbranched topology results in small coil dimension. Thus, the polyethylenes must be of very high molecular weight in order to thicken the lubricant when used as viscosity index improvers.
Hyperbranched polyethylenes have been synthesized previously by chain-walking polymerization catalyzed by late transition metal α-diimine catalysts. Due to the transitional state nature of the late transition metal catalyst and the requirement of low ethylene pressure to achieve hyperbranching, the molecular weight that can be achieved is limited. Raising the ethylene pressure can increase molecular weight, but branching density is severely compromised. Thus, attainment of both high molecular weight and hyperbranching has not been possible with the chain-walking polymerization mechanism. Difunctional acrylate crosslinking agents can be used to boost molecular weight in hyperbranched polyethylenes, but even a slight excess can result in gelation.
It would be desirable to have a method for making hyperbranched polyethylenes of high molecular weight.